June 01, 2013

Electric Sail components and technical details

The electric solar wind sail (E-sail) is a new type of propellantless propulsion system for Solar System transportation, which uses the natural solar wind for producing spacecraft propulsion. This paper discusses a mass breakdown and a performance model for an E-sail spacecraft that hosts a scientifi c payload of prescribed mass. In particular, the model is able to estimate the total spacecraft mass and its propulsive acceleration as a function of various design parameters such as the tethers number and their length. A number of subsystem masses are calculated assuming existing or near-term E-sail technology. In light of the obtained performance estimates, an E-sail represents a promising propulsion system for a variety of transportation needs in the Solar System.

A baseline, full-scale, E-sail propulsion system comprises 2000 km of total main tether length (for example 100 tethers, each one being 20 km long), with 25 kV tether voltage, 960W electron gun power consumption and 1.16 Newtons nominal thrust at 1 au from the Sun. If the main tethers are su fficiently long such that the potential sheath overlapping between them is negligible, the propulsive thrust varies as 1=r, where r is the Sun-spacecraft distance. Note, for comparison, that in the classical photonic solar sail the propulsive thrust decreases more rapidly (that is, as 1=r2) with the solar distance. Therefore the E-sail concept is especially attractive for a mission towards the outer Solar System, such as a Jupiter rendezvous or a mission towards the Heliopause and the Solar System boundaries.

Esail schematic

The E-sail propulsive thrust per unit length (of a main tether) is expectedly about 580 nN=m so that, for example, a 20 km long tether gathers about 11.6 mN of thrust from the surrounding solar wind plasma. The previous thrust estimate at 1 au corresponds to an average solar wind. Actually the solar wind properties vary widely along basically all relevant timescales. However, due to certain plasma physical eff ects, the E-sail propulsive thrust tends to vary much less than the solar wind dynamic pressure when a simple constant power strategy is applied to adjusting the tether voltage in response to solar wind density variations.

When the E-sail size increases, its specifi c acceleration improves and the E-sail mass fraction decreases. This is because the main tether reels and RUs have, by assumption, a certain base mass even in the limit of short main and auxiliary tethers.

By redesigning and miniaturizing these items, the E-sail speci c acceleration could probably be improved for small sizes. On the other hand, the trend would probably not continue to even larger sizes, because for tethers longer than 20 30 km, their tensile strength requirement would start to grow beyond what Heytethers tolerate. If even longer tethers were used, thicker wires or better materials should probably be employed.